The IEEE 802.11b standard has been widely adopted for use in wireless local area networks. In a typical 802.11b network, clients such as user PCs and laptops wirelessly communicate with access points linked to a fixed wired infrastructure. Such 802.11b access points and clients are widely deployed in homes, retail stores, hospitals, educational institutions, businesses, etc. The 802.11b standard exploits spectrum at 2.4 GHz and achieves a maximum physical layer data rate of 11 Mbps.
To increase data rate, a new standard has been developed, 802.11g. While 802.11b employs direct sequence spread spectrum (DSSS) and Complimentary Code Keying (CCK), the 802.11g standard employs orthogonal frequency division multiplexing (OFDM). The 802.11g standard is advantageous in that data rates as high as 54 Mbps are available. The 802.11g devices employ the same spectrum as the 802.11b devices. Accordingly, the IEEE 802.11 standards body incorporated certain features in the 802.11g standard to ensure that 802.11g devices are backwards compatible with 802.11b devices and that the two standards can coexist successfully.
Despite the backward compatibility and coexistence features inherent in the 802.11g standard, there are nonetheless several compatibility problems that degrade the throughput of wireless networks where the two types of devices are mixed. 802.11g-capable access points and clients are also capable of 802.11b operation. However, legacy 802.11b-only devices have no capability of transmitting or receiving 802.11g OFDM signals. Compatibility problems include the following:
802.11b-only clients cannot detect 802.11g OFDM transmissions. An 802.11b radio may therefore attempt to transmit during a current 802.11g OFDM transmission thus causing a collision. To prevent this type of collision, the 802.11g standard provides for a protection mode to be used by 802.11g-capable radios whenever one or more 802.11b-only radios are in the vicinity. In the protection mode the g-capable radios precede their transmissions with an RTS/CTS (request to send/clear to send) exchange or a CTS message transmission. The RTS and/or CTS messages are sent using the 802.11b modulation scheme and serve the purpose of notifying the 802.11b radio that the medium will be busy for a specified time. These messages, however, add a relatively large amount of overhead reducing wireless throughput. In a multiple access point network, a single b-only client can degrade throughput in multiple cells due to either collisions or the measures that need to be taken to prevent them.
Another problem is that g-capable clients can associate to b-only access points even when g-capable access points are nearby. The b-only access point may be selected, for example, because it has a higher received signal strength. Throughput is lost here because rather than sending data at 54 Mbps to and from the g-capable access point, the g-capable client will only send data at 11 Mbps to and from the b-only access point.
Single-cell simulations have been performed to examine the throughput for network scenarios providing only b-only clients, only g-capable clients, and mixed scenarios that incorporate both b-only clients and g-capable clients. The following table shows throughput possible within a single cell on a single channel evaluated at the medium access contention layer.
CONFIGURATIONSINGLE CELL THROUGHPUTAll 802.11b clients6 MbpsAll 802.11g clients22 Mbps Mixed 802.11g and 802.11b clients7 Mbps
It will then be seen that simply replacing an 802.11b access point with an 802.11g-capable access point and replacing a majority of 802.11b-only clients with 802.11g-capable clients provides very little improvement to network throughput.
What is needed are systems and methods for significantly improving the throughput of networks that employ mixed modulation types such as 802.11g and 802.11b.